The long-term objective of this proposal is to understand in quantitative detail the cAMP- and Ca2+-signaling in sensory transduction by olfactory receptor neurons (ORNs). We shall focus on the question of canonical olfactory-transduction mechanism in ORNs of the vertebrate main olfactory epithelium. The major chemo-transduction mechanism in ORNs of the main olfactory epithelium involves a cAMP-signaling cascade, leading to Na+ and Ca2+ influxes through a cyclic-nucleotide-gated (CNG), non-selective cation channel to depolarize the ORN to firing threshold. The Ca2+ influx leads to signal amplification via a Ca2+-activated Cl channel, as well as olfactory adaptation via multiple Ca2+-activated negative-feedback pathways. Recently, the significance of these amplification and negative-feedback pathways is nonetheless thrown into doubt and confusion. We propose to refine and quantify our understanding of the transduction process and to resolve the confusion surrounding the signal amplification and negative feedbacks. In Aim 1, we shall characterize the ORNs from the M71-monoclonal-nose mouse line, with the objective of using it as the common genetic background for many of the experiments in this proposal. This genotype offers the distinct advantage of having a near-homogeneous population of ORNs in the main olfactory epithelium, making experimental investigations much more efficient, and, in some cases, exclusively possible. In Aim 2, the objective is to carry out a detailed dissection, in both amplitude and time, of the overall ORN olfactory response into its CNG and Cl- current components for determining their correlations in amplitude and kinetics. In Aim 3, our goal is to re-examine the hypothesis that the Ca2+-mediated signal amplification via the Ca2+-activated Cl- current dictates the stimulus threshold for odorant detection. In Aim 4, our objective is to resolve the mystery/confusion about the relevance of the Ca2+-mediated negative feedbacks in olfactory transduction. Elucidating the steps of olfactory transduction will provide great insight into olfactory malfunctions arising from genetic defects in the transduction pathway, as amply demonstrated by the huge success in the case of vision.